Optical waveguides and, in particular, optical fibers, are often used for sensing changes in an ambient medium. Optical sensors have been used to measure changes in various parameters such as temperature, pressure, sound, refractive index and the like. In many cases, these changes are detected by monitoring the transmission (or reflection) spectrum of light as it propagates along an optical waveguide disposed within the ambient. Some optical sensors function as evanescent sensors based on the detection of changes in light propagating through an optical waveguide due to the optical mode that evanescently penetrates into the surrounding ambient.
Indeed, evanescent wave absorption is an effective technique for performing various types of environmental sensing. When a beam of light propagates along an optical fiber, the electromagnetic field does not abruptly fall to zero at the core/cladding interface. Instead, the overlap of an incoming beam and the internally reflected beam leads to a field that penetrates into the medium adjacent to the core region of the fiber. This electromagnetic field, which tails into the adjacent medium, is defined as the “evanescent field”.
In order to enhance the response of the transmission (or reflection) spectrum to variations of ambient medium parameters, an optical sensor is typically configured as a Mach-Zehnder interferometer (MZI) having at least two separate arms along which an optical signal will propagate. At a given wavelength λ, the output power of an N-arm MZI is determined by the following equation:
      P    =                                                ∑                          n              =              1                        N                    ⁢                                    A              n                        ⁢                          exp              ⁡                              (                                  ⅈ                  ⁢                                                                          ⁢                                      L                    n                                    ⁢                                      β                    n                                                  )                                                                2        ,where Ln is defined as the length of waveguide n and An and βn are the amplitude and propagation constants of the particular optical signal propagating along waveguide n. In the simplified case where n=2 and each arm has the same length L, the above equation reduces to the following relation:P=|A|2{1+cos [L(β1−β2)]}.In the analysis of an exemplary measured parameter q (where q may be, for example, temperature, refractive index, etc.), a variation in q causes variation in at least one of the propagation constants, say β1(q). From the above, it is clear that the sensitivity of the sensor is proportional to the following:
                        ∂        P                    ∂        q                  =      L    ⁢                        A        ⁢                                  ⁢                  sin          ⁡                      [                          L              ⁡                              (                                                      β                    1                                    -                                      β                    2                                                  )                                      ]                                      ⁢                                                ∂                          β              1                                            δ            ⁢                                                  ⁢            q                                      .      Thus, it is shown that the sensitivity grows proportionally to the length L of the MZI arm. For this reason, it is desirable to make the interferometer arm as long as possible. On the other hand, increasing the length L results in increasing the overall physical size of the sensor. The latter is undesirable for at least two reasons. First, this causes spatial delocalization of the measurement since the ambient may change over the length of the interferometer arm. Second, many applications require the use of a “miniature” sensor (for example, in a “lab on a chip” application).
It has previously been suggested to fabricate miniature MZI sensors based on photonic wires that are folded or spirally bent to be used as a planar photonic circuit. However, these devices are known to experience relatively high losses and cannot provide the degree of sensitivity required for many applications. Input/output coupling to/from these photonic wire devices is also problematic and introduces unwanted optical losses into the system.
Thus, a need remains in the art for a “miniature” optical sensor that exhibits the sensitivity generally associated with larger, multi-component arrangements.